An exhaust gas produced by incineration of wastes in an incinerator contains detrimental components such as hydrogen chloride and sulfur oxide. In addition, a fly ash collected from the exhaust gas contains detrimental heavy metals such as zinc, lead and cadmium. It is the conventional practice therefore to collect a fly ash from the above-mentioned exhaust gas discharged from the incinerator by means of a dust collector, then remove detrimental components such as hydrogen chloride and sulfur oxide contained in the exhaust gas by means of a wet type gas cleaner using, for example, caustic soda, and then release the thus treated exhaust gas through a chimney to the open air.
A typical chemical composition of the fly ash collected from the exhaust gas as described above is shown in Table 1.
TABLE 1 ______________________________________ (wt. %) SiO.sub.2 CaO Fe.sub.2 O.sub.3 Al.sub.2 O.sub.3 SO.sub.3 MgO Na K Cl ______________________________________ 15-25 8-20 3-7 12-21 4-8 2-6 3-8 3-9 9-16 ______________________________________ [mg/Kg] Zn Cd Pb Total Cr Total Hg ______________________________________ 4000-20000 150-500 1500-7000 230-570 2.3-7.5 ______________________________________
For such a fly ash, which is usually dumped to a land site for reclamation, it is necessary prior to dumping to apply a treatment to make insoluble detrimental heavy metals contained in the fly ash.
For the purpose of making insoluble detrimental heavy metals contained in the fly ash, the following methods are known:
(1) A method comprising adding cement and water to the fly ash and kneading the mixture into a lump;
(2) A method comprising adding molten asphalt to the fly ash and kneading the mixture into a lump;
(3) A method comprising placing the fly ash in a tank containing water, neutralizing the mixture by means of an exhaust gas from an incinerator blown into the tank to convert heavy metals in the fly ash into insoluble compounds, then removing water, and forming the insoluble compounds into a lump; and
(4) a method comprising adding an acid and a sulfide to the fly ash to convert heavy metals into insoluble compounds, then removing water, and forming the insoluble compounds into a lump.
The method (1) above of forming the fly ash into a lump by means of cement has the following problem: because of the high alkalinity of cement, after dumping heavy metals such as lead, which are easily soluble into a liquid having a high pH value, may be dissolved from the lumpy fly ash under the effect of rain water, for example. An increased amount of added cement results in the dissolution of salts contained in the fly ash in rain fall, thus making the lumpy fly ash porous. The lumpy fly ash tends therefore to easily collapse.
The method (2) above of forming the fly ash into a lump by means of molten asphalt has the following problem: when the fly ash has a high alkalinity, after dumping, heavy metals may be dissolved from the lumpy fly ash as described above under the effect of rain water.
The method (3) above of treating heavy metals contained in the fly ash through neutralization has the following problem: the steps for neutralizing, dewatering and treating waste water produced in dewatering require complicated facilities, thus leading to a high cost.
The method (4) above of treating heavy metals contained in the fly ash by means of an acid has the following problem: the steps for acid treatment, dewatering and treating waste water produced in dewatering require complicated facilities, consuming much acid, thus resulting in a high cost.
Under such circumstances, there is a strong demand for the development of a method for making insoluble heavy metals contained in a fly ash, as collected from an exhaust gas discharged from an incinerator, easily and economically and stably for a long period of time, but such a method has not as yet been proposed.